1. Field of the Invention
The present invention generally relates to recording data modulating apparatus and reproducing data demodulating apparatus for use in recording and/or reproducing digital data. More particularly, this invention relates to recording data modulating apparatus and reproducing data demodulating apparatus in which a modulation code is a variable length code for converting recording data of m-bit unit into a recording code of n-bit unit and to a reproducing data demodulating apparatus in which a variable length code and a maximum likelihood decoding such as a Viterbi algorithm are combined.
2. Description of the Prior Art
Recently, in recording and reproducing appliances such as a digital VTR (video tape recorder) or the like, advance in high density has been made and as one of its elemental techniques, a variety of modulation and demodulation systems are examined. Particularly, there is known a partial response (Partial Response) system or the like which positively utilizes an intersymbolic interference from the beginning in order to restrict the intersymbolic interference and performs a code configuration.
Particularly, Kretzmer (E. R. Kretzmer), a proposer of this system, mentioned five kinds of characteristics as typical ones of a transmission system upon a dual information series and referred to them as classes I, II, III, IV and V, respectively. Among these classes, FIG. 1 shows a frequency characteristic of the class I (hereinafter referred to as Pr (1, 1)) of a so-called duobinary code (Duobinary) used in the present embodiment. Such Pr (1, 1) enables the frequency characteristic to be compressed into 1/2 in comparison with a single-current system encoding. In order to restrict a recording band in this way, and, further, to reduce deterioration of S/N ratio, a decoding system called a maximum likelihood decoding (Maximum Likelihood Sequence Estimation) is also employed.
A Viterbi decoding is known as the simplest system of the maximum likelihood decoding. The Viterbi decoding is a decoding method which permits the whole electric power of an input signal to be effectively used in the same way in a matched filter. A fundamental construction of such viterbi decoding and a construction of a viterbi decoding circuit or the like upon application thereof to the recording and reproducing system in the form of an NRZI code are described in detail by Yoshizumi Eto and two others, in "A digital video recording technique", Nikkan Industrial Newspapers Co., pp 72-84, Aug. 31, 1990.
There is known "data conversion and detecting method" which is disclosed in Japanese Laid-Open Patent Publication No. 4-76867 (laid-open on Mar. 11, 1992). According to this prior art, by increasing a pattern distance between code patterns in order to avoid an intersymbol interference, it is possible to increase a recording density while a present recording medium and recording and reproducing device are utilized. According to the arrangement of this prior art, in a data converting circuit in the recording system, of N codes of conversion data, n consecutive codes the number of which is equal to the number of intersymbol interferences that can be allowed in view of the characteristic of transmission path are weighted so as to linearly decrease from a reference value of a distribution center. When n consecutive codes thus weighted are sequentially added to form intermediate series of N-n+1 bits, conversion data is selected as a code so that a total of absolute values of a difference between codes of the intermediate series becomes more than a predetermined number of times the value of a reference value of the weighting. Thus, the pattern distance between the code patterns is increased.
Further, there is known "data conversion and detection method" which is disclosed in Japanese Laid-Open Patent Publication No. 4-89664 (laid-open on Mar. 23, 1992). In this prior art, as a code of at least a preceding portion of conversion data, there is selected one such that a total (modulation code distance) of absolute values of a difference between codes of intermediate series becomes more than a predetermined number of times the standard value of the weighting. Thus, an error rate in the playback can be reduced and a recording density can be improved. An arrangement of this prior art will be described below. N codes of conversion data are divided into Ns preceding portions and Nt succeeding portions. Of Ns preceding portions and Nt succeeding portions, n consecutive codes whose number is equal to the number of intersymbol interferences that can be allowed in the transmission path are weighted so as to decrease linearly from the center. When n consecutive codes thus weighted are sequentially added to form intermediate series, as at least the code of the preceding portion of conversion data, there is selected one such that a total of absolute values of difference between codes of intermediate series becomes more than a predetermined number of times the standard value of the weighting. Therefore, the pattern distance between the code patterns can be increased and the error rate in the playback mode can be reduced while the present media and recording and reproducing device are utilized. Also, the recording density can be improved.
Furthermore, there is known "Application of New Variable-length block code and Viterbi Decoding using d-constraint to magneto-optical-Recording", Vol. 44, No. 10, pp. 1369 to 1375, 1990, in The Journal of the Institute of Television Engineers of Japan. A summary of this paper will be described below. A (3, 19; 4, 9;3) code, one of the variable-length block codes, is developed. This code introduces less intersymbol interference into magneto-optical recording than a conventional (2, 7; 1,2;4) code. A PR(1, 1) system, used for signal detection, and Viterbi decoding using the d-constraint of these codes as a decoding method, are proposed. Their error probabilities are obtained by computer simulation. Their performance is compared with the conventional peak detection method. The results show the proposed method exhibits excellent performance compared with the conventional one, and a large improvement in performance is achieved by using a (3, 19; 4, 9; 3) code.
The above-mentioned digital video recording technique discloses (pp. 83 to 84) the technique that the Viterbi decoding is applied to the Pr(1, 1) transmission path (optical disc), whereby an error rate is improved and a signal-to-noise (S/N) ratio is improved by 2 dB.
Although the S/N ratio can be improved and an electric power can be utilized effectively by using both the maximum likelihood decoding and the partial response system, this maximum likelihood decoding is a sequential decoding. Thus, when a predetermined metric is detected, a point immediately preceding the detection point must be detected and therefore the parallel processing cannot be effected.
Further, when the calculation for detecting a metric is carried out, there is then the probability that there remain indefinite data. Therefore, the length to the end that data is determined cannot be determined as a predetermined length. Consequently, the hardware arrangement becomes complex and the circuit scale is increased.
Furthermore, there occurs an error propagation (see pp. 79 to 81 of the above digital video recording technique). There is then the possibility that this error occurs over 2 symbols. In such case, there are then the problems that an error correction code or the like will be destroyed.